Thermal power engine and its operating method

Abstract: A method for improving the total production of useful energy in an energy utilisation system of a thermal power engine (1) that is liquid-cooled and is used for the production of thermal energy as well as mechanical energy. In the utilisation system thermal energy is taken from the coolant of the engine cooling system. At least a part of the coolant from the engine is led to a vaporisation area (5) where, either by lowering the pressure or by increasing the amount of thermal energy within that area (5), a part of the coolant is transformed to vapour. The vapour is used within the energy utilisation system for energy transport and/or as a medium for recovering energy.

Description: The invention relates to a method according to the preamble of claim 1 for improving the total production of useful energy of an internal combustion engine producing heat and mechanical energy, herein sometimes referred to as a "thermal power engine" and to an engine intended for the application of this method. When using some thermal power engines, such as for example large diesel engines, a significant part of the energy content of the fuel consumed will not be used for useful purposes, because, in the cooling system of the engine, it only raises the temperature of the coolant to a level of some 80 to 85°C and it is difficult to find a useful application for waste heat at such low temperature values. In some rather large plants employing thermal power engines the thermal energy of the coolant has however been used, by means of heat exchangers, to produce, for example, warm water (e.g. warm process water) or for district heating applications. JP-A-5853608 discloses a known waste heat utilising system in which cooling water from an engine is supplied to a flush tank where steam is generated. No mechanism is disclosed for maintaining a higher coolant pressure in the engine than in the tank. Furthermore the system does not disclose an energy efficient system in which the cooling system remains in balance. An aim of this invention is to create a thermal power engine, for example a diesel engine, in which it is possible to utilise the heat energy produced in a more efficient manner. The aim of the invention is achieved by the means stated in claim 1. By vaporising for example some of the coolant used in a diesel engine, a part of its thermal energy is absorbed as heat of vaporisation. Vapour is a medium, the contained thermal energy of which can be quite effectively utilised by known methods. The partial vaporisation is obtained either by lowering the pressure or by bringing more thermal energy to the vaporisation area. This takes place preferably in a separate area outside the thermal power engine, but vaporising may also theoretically take place by using heat produced by the engine in the cooling system of the engine, although this method of operating requires that the vaporisation process takes place in such a part of the engine and under such good control that it is not harmful or dangerous to the functioning of the engine. It is of advantage to build thermal power engines intended to operate according to the invention as rather large units, having normally an output power of the order of magnitude of some megawatts, even some tens of megawatts. The greater is the power of a thermal power engine or plant containing such engines, the more important for energy saving it is to find a useful application for its waste heat and the better are the chances that the investments made for waste heat recovery will prove to be profitable. A thermal power plant may comprise several thermal power engines, whereby the total power output of the plant may be of the order of magnitude of 100 MW. In known large internal combustion engines, the temperature of the coolant for the engine is often less than 100°C, usually in the range 80°C to 85°C. In a thermal power engine operating according to the invention, it is of advantage to operate with a coolant temperature above normal for thermotechnical reasons. This requires, however, that the thermal power engine be able to withstand coolant temperatures above normal in its cooling system and that the temperature of the coolant is properly supervised and controlled. What constitutes the method of the invention is set out in the following claim 1. By superheating vapour by using different waste heat sources associated with a thermal power engine, for example by means of the exhaust gases, the usefulness of the vapour for practical purposes is improved. Vapour, in particular superheated vapour, may be used for many purposes, for example as a process vapour in different machines and devices or for the production, (e.g. in a steam turbine), of mechanical energy which can easily be converted to electric energy. It is known to improve the utilisation of vapour by condensing the vapour at very low pressure in a condenser and returning it, in liquid state, to a vaporisation circuit. In accordance with the invention this known technique is applied to the cooling system of a thermal power engine. By preheating the returning condensate, with waste heat taken from the thermal power engine, a requisite is created either for its revaporisation before return to the cooling system or for use of its available thermal energy by feeding it into the coolant circuit from where it may be further utilised. A part of the cooling medium returned to liquid phase may be led, before its return to the cooling system, to a second circuit where a fresh vaporisation is accomplished using the heat content of the flow of some hot medium available from the thermal power engine. In this way at least a part of the vapour produced may be fed into the vapour flow emanating from the first vaporisation phase before any possible superheating of the vapour has been undertaken. A part of the vapour of the second circuit may also form a separate circuit passing through a turbine and a condenser. These types of circuits open up the possibility of utilising a heretofore unused part of the waste heat of a thermal power engine in a coolant circuit according to the invention. By keeping the pressure of the cooling system of a thermal power engine at a level of more than 2 bar, and preferably more than 5 bar, it will be possible, when using pure water as a coolant, to raise its temperature considerably over 100°C without causing boiling. For applying the invention, it is believed that pure water is the best coolant. If its pressure is kept so high that the temperature in the cooling system may exceed 140°C, good conditions prevail for utilising, according to the invention, the thermal energy of the coolant. The invention may, with advantage, be applied to a large supercharged diesel engine, in which some high temperature medium flow of the supercharging arrangement of the engine can be used for superheating vapour received from the coolant of the engine and/or for revaporising condensed coolant. Thereby a substantial part of the waste heat of the supercharging arrangement can also be recovered. The high temperature charge air of the supercharging arrangement may be used for superheating vapour and/or for vaporising condensate, whereby at the same time cooling of the charge air is achieved, which allows the operating efficiency of the engine to be optimised. In a method according to the invention, the energy required for the vaporisation of the coolant largely corresponds to the amount of thermal energy transferred to the coolant from the thermal power engine. Thus a thermal balance is achieved and at the same time the waste heat from the cooling of the engine is effectively utilised. According to another aspect of the present invention there is provided an engine plant as claimed in the ensuing claim 11. The invention will now be further described in greater detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a schematic diagram of one arrangement according to the invention, Figure 2 shows a flow diagram of another embodiment of the invention, and Figure 3 shows a graph of the shaft power of a steam turbine utilising waste energy from a thermal power engine, as a function of absolute vapour pressure in three different operating modes. In Figure 1, 1 indicates a thermal power engine, for example a large water-cooled supercharged diesel engine. The cooling system of the engine 1, includes a pipe 2 which has a pressure and temperature monitoring control device 3 operating a valve 4 for controlling the pressure in the coolant (cooling water). The cooling water is led to a tank 5, defining a vaporisation space, in which the pressure is so much lower than in the coolant passages in the engine 1, that a part of the cooling water (typically under 5%) is transformed into steam. That part of the cooling water (usually over 95%), which does not become vaporised, is, by means of the pump 7, pumped through a pipe 8 back to the coolant passages of the engine 1. The level of liquid in the tank 5 is monitored by a control device 9, which controls the return flow of water in a pipe 21 by means of a throttle valve 6. The pressure in the cooling system of the engine 1 and in the tank 5 is preferably so selected that the pressure of the steam generated is from 4 to 8 bar, in the embodiment shown about 6 bar, the temperature being correspondingly about 159°C. The steam is led through a pipe 10 to a superheater 11 in the exhaust line 23 of the engine 1, in which its temperature rises to about 300°C. From the superheater 11 the steam is led through a pipe 12 to a steam turbine 13, in which the steam is used to generate mechanical energy, which, as illustrated , is used for producing electricity in a generator 14. From the turbine 13, low pressure steam (s 0.1 bar) flows through a pipe 15 to a condenser 16, from which the condensate water is led, through a pipe 17, to a pump 18. The pump 18 raises the pressure of the water to such a level that the water, after passing through a preheater 22 also provided on the exhaust line 23, the pipe 21 and the throttle valve 6 is able to join the water flow in the pipe 8 on the suction side of the pump 7. Possible leakage losses are replaced by water fed through a pipe 19 upstream of the pump 18. In the superheater 11, in the preheater 22 and in an exhaust gas vaporiser 31, thermal energy is obtained from the hot exhaust gases of the engine 1, which are led away from the engine through the exhaust gas line 23. Downstream of the heat exchanger 22, the temperature of the water is about 159°C. A part of the preheated water is, in the embodiment according to Figure 1, led through a pipe 25 to a tank 26 which functions as a vapour separator and from which the water separated out is led through a pipe 27 to two branches 28 and 34. Branch 28 passes, via a pump 29, to the exhaust gas vaporiser 31 and branch 34 passes, via a pump 35, to a charge air vaporiser 37 heated by compressed charge air flowing to the engine 1 through a pipe 39. The branches 28 and 34 unite again in the tank 26, from which the steam separated out is led back to the pipe 10. The supercharging arrangement of the engine 1 comprises an exhaust gas turbine 38 and a charge air compressor 40 driven by the turbine 38. The temperature of the supercharged air may rise close to 250°C. With respect to efficiency considerations of engine operation, it is important to cool the charge air and the required cooling may at least partly take place as shown using the vaporiser 37. Further air cooling in a cooler 42 is usually required, which lowers the temperature of the charge air to a desired centigrade value. Figure 2 has much in common with Figure 1 and like reference numerals have the same meaning in both Figures. In the embodiment according to Figure 2, the superheater 11 has been replaced by a twin superheater 11a, which has two separate steam flow ducts. The circulation path through the twin superheater 11a, which includes the pipe 12, the steam turbine 13, the condenser 16 and the preheater 22 corresponds to the arrangement shown in Figure 1. Downstream of the preheater 22, a part of the preheated water is led through a pipe 25 to two separate tanks 26a and 26b, which both operate as steam separators. From the tank 26a the water is led through a pipe 34 and a pump 35 to a charge air vaporiser 37 from which the generated steam-water mixture is led back to the tank 26a and there, further, to the pipe 10 just as shown in the embodiment of Figure 1. From the tank 26b, the water is led through a pipe 27 and a pump 29 to an exhaust gas vaporiser 31, from which the steam-water mixture generated, as in the embodiment according to Figure 1, is led back, but now to the tank 26b, from which the steam flows through the second flow duct of the twin superheater 11a and through a pipe 12a to a second steam turbine 13a and there produces mechanical energy. The low pressure steam (≤ 0.1 bar), received from the steam turbine 13a flows through a pipe 15a to the condenser 16, in which it unites with the low pressure steam flow coming from the steam turbine 13. The use of two separate steam turbines is justifiable, because the temperature and the pressure of the steam, received from the exhaust gas vaporiser 31 are notably higher than the temperature and pressure of the steam, received both from the charge air vaporiser 37 and from the tank 5. In the graph of Figure 3, curve A presents, as a function of the steam pressure p (in bar), the shaft power P (in kilowatts) of a steam turbine utilising the waste heat of a diesel engine when the steam turbine recovers thermal energy only from the exhaust gases of the engine. Curve B in Figure 3 represents the shaft power of the turbine as a function of the steam pressure, when a part of the thermal energy of the liquid coolant circulating in the engine is vaporised and used according to the invention for useful purposes with the arrangement shown in Figure 1 but without the charge air vaporiser 37 and its circulation arrangement (i.e. without using the circuit 26, 27, 34, 35, 37 and 26). Curve B shows that the shaft power of the steam turbine rises some 30 to 40% compared to the application represented by curve A. However, the relationship between the power of the steam turbine and the steam pressure is not a convenient one in curve B because it is difficult to achieve the high steam pressures which correspond to maximum power output. Thus, the most advantageous areas of curve B will not easily be attainable in practice. Curve C corresponds to an application according to Figure 1 but using all the heat utilisation arrangements shown. It is considerably more advantageous than that represented by curve B, because now the peak power occurs within the pressure range 4 to 6 bar, which is a realistic pressure range for utilising the invention. The arrangement of Figure 2 gives a curve similar to curve C. It should be noted that all pressure values given herein relate to absolute pressure and this is the explanation for the "a" in the pressure units quoted in Figure 3. The invention is not restricted to the embodiments illustrated, since several modifications are feasible within the scope of the following claims.

Claims: A method for improving the total production of useful energy in an energy utilisation system of a thermal power engine (1) that is liquid-cooled and is used for the production of thermal energy as well as mechanical energy, in which system at least a part of the coolant from the engine is led to a vaporisation space (5), where either by lowering the pressure or by increasing the amount of thermal energy within that space (5) a part of the coolant is transformed to vapour, and the vapour generated from the coolant is superheated (in 11, 11a) by means of a hot fluid flow (in line 23) of the engine (1), the coolant vapour being used within the energy utilisation system for energy transport and/or as a medium for recovering energy, characterised in that the pressure of the coolant in the engine is maintained higher than the pressure in the vaporisation space (5) such that the coolant in the engine is liquid, and in that the amount of energy required for the vaporisation of the coolant in the vaporisation space (5) substantially corresponds to the amount of thermal energy transferred to the coolant from the thermal power engine during cooling of the latter, whereby a thermally balanced cooling and heat utilisation arrangement is achieved. A method according to claim 1, characterised in that coolant transformed to vapour is used in a steam turbine (13, 13a) for producing useful mechanical energy. A method according to claim 1 or 2, characterised in that vaporised coolant taken from the cooling system (2, 7, 8) of the engine (1) is retransformed to liquid phase (in 16) and is returned in liquid state to the cooling system of the engine, preferably, so that the liquid is preheated (in 22) by thermal exchange with some hot fluid flow of the engine before being returned. A method according to any one of the preceding claims, characterised in that a part of the coolant that has been liquefied and is to be returned to the cooling system (2, 7, 8) of the engine (1) is led to a second circuit (25, 26-29, 31), in which, by means of some hot fluid flow of the engine (1), revaporisation is performed, from which at least a part of the vapour generated is led to the vapour flow from the vaporisation area (5) before any possible superheating of that vapour flow. A method according to claim 4, characterised in that a part of the vapour from the second circuit (25, 26) is fed to a separate circuit (11a, 12a) through a turbine (13a) and a condenser (16). A method according to any one of the preceding claims, characterised in that the pressure of the liquid coolant in the cooling system (2, 7, 8) of the engine (1) is maintained at a level of more than 2 bar, preferably of more than 5 bar. A method according to claim 6, characterised in that in the cooling system (2, 7, 8) the pressure of the coolant is kept sufficiently high, that the temperature of the coolant may, without boiling, considerably exceed 100°C and preferably exceed 140°C. A method according to any one of the preceding claims when applied to a supercharged diesel engine (1), which is characterised in that some high temperature fluid flow of the supercharging arrangement is used (in 37) for superheating the vapour received from the coolant of the engine and/or for revaporising condensate from that coolant. A method according to claim 8, characterised in that high temperature charge air (in 39) is used for vaporisation of condensate (in 37) and/or for superheating vapour, whereby at the same time cooling of the charge air is achieved. A method according to any one of the preceding claims, characterised in that the energy required for vaporisation of coolant is substantially wholly taken from thermal energy transferred to the coolant from the engine (1). An engine plant comprising a liquid-cooled thermal power engine (1) used for the production of thermal and mechanical energy and an energy utilisation system connected to receive coolant from the engine (1) and which comprises means (5) defining a vaporisation space for receiving coolant from the engine, means for lowering the pressure within, or increasing the thermal energy within, the vaporisation space to transform to vapour a part of the coolant supplied to the vaporisation space, superheater means (11) for superheating the coolant vapour by thermal exchange with a hot fluid flow of the engine and means for using the coolant vapour formed in the vaporisation space for energy transport and/or as a medium for recovering energy from the coolant vapour, characterised in that the energy utilisation system further comprises means for maintaining a higher coolant pressure in the engine (1) than in the vaporisation space such that the coolant in the engine is liquid, and in that the amount of energy required for the vaporisation of the coolant in the vaporisation space substantially corresponds to the amount of thermal energy transferred to the coolant from the thermal power engine during cooling of the latter, whereby a thermally balanced cooling and heat utilisation arrangement is achieved.

Inventors: AHNGER ANDERS, BAECKMAN KAJ, STORHOLM STEFAN, AHNGER, ANDERS, BAECKMAN, KAJ, STORHOLM, STEFAN, Bäckman, Kaj
Applicants: WAERTSILAE NSD OY AB
Application year: 1994
Main classification: F02G5
IPC codes: F01K23, F02G5, F02B3